Behind a black fence that blocks the view from outside, the school in Newtown, Connecticut, where 20 first-graders and six employees were killed by a 20-year-old gunman in December, is being razed.

Robert Mitchell, chairman of the town's public building and site commission, said Friday that demolition of one section of the school, Sandy Hook Elementary, began on Thursday and continued on Friday.

Photographs and video footage from news helicopters that flew overhead showed backhoes ripping off the roof.

Tragic site: Workers tear down part of Sandy Hook Elementary School in Newtown, Connecticut. Photo: Reuters

Mr Mitchell said no one had been shot in the section that was torn down this week.

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Adam Lanza, the gunman who opened fire at the school on December 14, wearing combat gear, had killed his mother at home before he went to the school.

The demolition would continue, section by section, for several weeks, Mr Mitchell said, and would be completed by the first anniversary of the shootings.

He said demolition workers prepared other parts of the single-story building to come down as they levelled the first section.

Officials had declared the school off-limits to everyone except the contractors hired to demolish it.

Mr Mitchell said the workers had been directed to destroy anything that could be identified with the school – in part to protect the victims' privacy, in part to prevent pieces of the school from being sold online.

The bricks would be crushed beyond recognition, he said, and the contractors were required to sign confidentiality agreements intended to keep descriptions of the inside of the school from circulating on the internet.

"We don't want to have happen what happened to some of the stuff from 9/11," he said.

"It would be embarrassing. The steel will be melted, and nothing identifiable will be leaving the site."

The town voted on October 5 to accept nearly $US50 million ($52 million) in state money for a new school to be built on the same site. The vote came five months after a task force recommended demolishing the old building.

More than 400 students are attending classes in a school building in nearby Monroe, that had been empty.

Mr Mitchell said architects were drawing plans for the new school in Newtown, which is expected to be completed in time for the 2016-17 school year.